A wireless device (also referred to as a user equipment, UE) transmits one or more uplink reference signals in a wireless communication system for any number of reasons, such as to permit the receiving base station to estimate the wireless channel. The wireless device typically generates a reference signal using one or more pseudo-random sequence generators. Accordingly, initialization of the sequence generator(s) with particular initialization sequence(s) dictates the uplink reference signal that the device transmits. The base station governs the initialization of the device's sequence generator(s) in this regard, meaning that signaling an initialization sequence to a wireless device presents challenges in terms of signaling overhead.
Consider, for instance, Long Term Evolution (LTE) networks. LTE networks are designed with the aim of enabling optional CoMP (Coordinated multipoint processing) techniques, where different sectors and/or cells operate in a coordinated way in terms of, e.g., scheduling and/or processing. An example is uplink (UL) CoMP where the signal originating from a single UE is typically received at multiple reception points and jointly processed in order to improve the link quality. UL joint processing (also referred to as UL CoMP) allows transformation of what is regarded as inter-cell interference in a traditional deployment into a useful signal. Therefore, LTE networks taking advantage of UL CoMP may be deployed with a smaller cell size compared to traditional deployments, in order to fully take advantage of the CoMP gains.
The LTE UL is designed assuming coherent processing, i.e., the receiver is assumed to be able to estimate the radio channel from the transmitting UE and to take advantage of such information in the detection phase. Therefore, each transmitting UE sends a reference signal (RS) associated with each UL data or control channel (e.g., PUSCH and PUCCH). 3GPP TS 36.211 V10.4.0 (2011-12), “Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 10).” In case of PUSCH, one demodulation reference signal (DMRS) per slot is transmitted on the same bandwidth as the uplink data channel. In case of PUCCH, multiple PUCCH-RSs are transmitted and time multiplexed by the UE within each subframe, spanning the PUCCH bandwidth assigned to the UE.
Additional RSs possibly transmitted by UEs consist of sounding reference signals (SRS). These reference signals are transmitted by a UE at predetermined time instances and over a predetermined bandwidth, in order to enable estimation of the UL channel properties at the network side.
RSs from different UEs within the same cell potentially interfere with each other and, assuming synchronized networks, even with RS originated by UEs in neighboring cells. In order to limit the level of interference between RSs, different techniques have been introduced in different LTE releases in order to allow orthogonal or semi-orthogonal RSs. The design principle of LTE assumes orthogonal RS within each cell and semi-orthogonal RS among different cells (even though orthogonal RSs can be achieved for aggregates of cells by so called “sequence planning”). However, orthogonality of DMRS transmitted by UEs belonging to different cell is currently under discussion in Rel-11 LTE standardization. A family of techniques for inter-cell DMRS orthogonality has been discussed. Some of these techniques rely on the possibility of coordinating the base-sequence index (BSI) employed for RS generation by different UEs in different cells, as described more fully later.
Another application in the UL of LTE is multi-user, multiple-input multiple-output (MU-MIMO), where data transmissions on PUSCH from multiple UEs are coscheduled on at least partly overlapping bandwidth in the same subframe, within the same cell. The UEs are separated at the receiver side by exploiting multiantenna processing. In order to allow the receiver to resolve the signals from the coscheduled UEs, it is beneficial to assign the DMRS in an orthogonal fashion for such UEs. This may be achieved by assigning different orthogonal cover codes (OCCs) to the DMRS of the coscheduled UEs. If the coscheduled bandwidths are fully overlapping, cyclic shift (CS) separation of the DMRS for the different UEs may also be exploited.
Each DMRS is characterized by a group-index and a sequence-index, which define the so called base-sequence index (BSI). BSIs are assigned in a cell-specific fashion in Rel-8/9/10 and they are a function of the cell-ID, where a cell-ID characterizes a cell in LTE and affects several cell-specific algorithms and procedures. Different base sequences are semi-orthogonal, which implies that some inter-sequence interference is present in the general case. The DMRS for a given UE is only transmitted on the same bandwidth of PUSCH and the base sequence is correspondingly generated so that the RS signal is a function of the PUSCH bandwidth. For each subframe, 2 RSs are transmitted, one per slot. In Rel-11 it is likely that UE-specific assignment of BSIs will be introduced.
Orthogonal DMRS can be achieved by use of cyclic shift (CS) in Rel-8/9 or by CS in conjunction with orthogonal OCC in Rel-10. CS is a method to achieve orthogonality based on cyclic time shifts, under certain propagation conditions, among RS generated from the same base sequence. Only 8 different CS values can be dynamically indexed in Rel-8/9/10, even though in practice less than 8 orthogonal DMRS can be achieved depending on channel propagation properties (without considering OCC in this example). Even though CS is effective in multiplexing DMRSs assigned to fully overlapping bandwidths, orthogonality is lost when the bandwidths differ and/or when the interfering UE employs another base sequence.
In order to increase interference randomization between different UEs (e.g., at different cells), a pseudo-random offset to the CS values is applied (CS hopping, CSH). The randomization pattern is cell-specific in Rel-8/9/10. A different CS offset is in general applied in each slot and it is known at both UE and eNB sides, so that it can be compensated at the receiver side during channel estimation. A CSH is generated according to a sequence initialization parameter cinit having 31 bits.
OCC is a multiplexing technique based on orthogonal time domain codes, operating on the 2 RS provided for each UL subframe. The OCC code [1 −1] is able to suppress an interfering DMRS as long as its contribution after the matched filter at the receiver is identical on both DMRSs of the same subframe. Similarly, the OCC code [1 1] is able to suppress an interfering DMRS as long as its contribution after the eNB matched filter has opposite sign respectively on the two RSs of the same subframe. It is straightforward to assume that CS and OCC will be supported also by Rel-11 UEs.
While base-sequences are assigned in a semi-static fashion, CS and OCC are dynamically assigned as part of the scheduling grant for each UL PUSCH transmission. Even though joint processing techniques may be applied for PUSCH, channel estimates based on DMRS are typically performed in an independent fashion at each reception point, even in case of UL CoMP. Therefore, it is crucial to keep the interference level at an acceptably low level, especially for RSs.
In case of SRS, the RSs are also generated according to a BSI (which may differ from the DMRS BSI for some UEs). Different SRS may be multiplexed by use of CS and COMBs. A COMB indicates a specific interleaved mapping of the RS to a subset of subcarriers. SRS assigned to different COMBS (i.e., non overlapping sets of subcarriers) are thus ideally orthogonal.
In case of PUCCH-RS, one or more RS per slot are generated, depending on the PUCCH format and other parameters. PUCCH-RS for different UEs are separated by use of CS and OCC, which spans over each slot. Also PUCCH-RS are generated according to a BSI that may in general differ from the DMRS BSI.
One of the improvements being discussed in LTE Rel-11 consists of the possibility of configuring the parameters for BSI and CSH initialization in a UE specific fashion, either semi-statically or dynamically, e.g., by signaling in the scheduling grants. Such configurability allows additional RS allocations options enabling, e.g., inter-cell orthogonality between UEs. R1-121028—“Details about UL DMRS configuration and signaling.” In order to achieve orthogonality by OCC, it is necessary to configure the paired UEs with the same CSH pattern. Problematically, however, the CSH initialization cinit is a 31 bit parameter, requiring significant overhead for being signaled.